HtEMATOPUS. 297 



both north and south of the equator, both in the New and Old Worlds ; winter visitors to 

 the tropics of both hemispheres. 



There is abundant evidence in support of the hypothesis that the centre of dispersion Original 

 of the Charadriidce was the North Pole ; and that the Oystercatchers were originally home - 

 residents on the shore of the Polar Sea, whence they were dispersed and driven southwards 

 by the arctic ice which formed and reformed during the Post-Pliocene Glacial Epoch. 

 Isolated on various distant shores, the Oystercatchers were differentiated into several 

 specific and subspecific forms. It is very interesting to trace their wanderings and to note 

 their specific differences, which agree in a remarkable manner with the hypothesis of their 

 polar origin and with their present geographical distribution, although the latter bears no 

 sort of relation to the geographical regions laid down by Messrs. Sclater and Wallace, 

 which refer to Passerine birds, but are seldom recognized by those belonging to other 

 families. 



When the descent of an arctic climate upon the North Polar regions drove the resi- Dispersal. 

 dents of the Polar Basin into more southern latitudes, coast-birds like the Oystercatchers 

 could escape by four routes : they could either follow the European or the American 

 shores of the Atlantic, or the Asiatic or American shores of the Pacific. If they availed 

 themselves of all four routes, the emigrants must soon have become isolated in four colonies, 

 which eventually produced four species or groups of species. Behring's Straits lies 25° 

 south of the Pole, and the Asiatic coast is connected by a row of islands with the Pacific 

 coast as far as 40° from the Pole, so that isolation, and consequently differentiation, began 

 late with the Pacific birds ; whilst in the Atlantic the emigrants were probably effectually 

 isolated for ever by Greenland's icy mountains at a distance of not more than 5° or 7° from 

 the Pole. It is consequently reasonable to expect that the Pacific Oystercatchers should be 

 nearer allied to each other than the Atlantic Oystercatchers are. 



The facts that the European Oystercatcher (H. ostralegus) is a resident in Iceland, Emigration 

 has been known to visit Greenland, and is a regular summer visitor to the coast of Lapland, along Atlun- 

 seem to prove that the group to which it belongs left the Polar Basin in two streams or Europe. 

 lines of emigration — one following the eastern shores of Greenland, crossing Iceland, the 

 Faroes, and the coasts of Great Britain to the coasts of West Europe, where it coalesced 

 with the other stream of birds which followed the coast of Scandinavia. 



The West Coast of Africa south of Morocco appears to have been of such a desert 

 character that the Oystercatchers were afraid to emigrate further in that direction. The 

 basin of the Mediterranean, on the other hand, provided the necessary outlet for the surplus 

 population, and the stream of emigration continued to flow eastwards from sea to sea, lake 

 to lake, and river to river, at least as far as the Obb. The advanced party appear to have 

 pushed forward still more to the east, and to have reached the valley of the Amoor, whence 

 they never returned, but, following the course of that river to its mouth, they established 

 an independent colony. The cessation of interbreeding between the birds of this colony 

 and the parent stock soon caused a variation between them, and the eastern birds are 



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